Casting mold and method of making same



Jan. 11, 1966 w. RUBEL ETAL CASTING MOLD AND METHOD OF MAKING SAME Filed Feb. 26, 1965 lOb Fig.2

INVENTORQ. WERNER RUBEL GERT DEVENTER BY j l g was: 5 Jissfzm United States Patent fitice 3,228,074 Patented Jan. 11 1966 4 Claims. (a. 22-193 Our present invention relates to a casting mold and, more particularly, to so-called permanent molds for the casting of liquid metals as well as to methods of making molds of this type.

Earlier techniques in the foundry art include the use of molding sands which are compacted about a pattern to form separable molds which, upon removal of the pattern, are charged with liquid metal. Sand molds of this type have the advantage that they are pervious to gases generated during casting by, say, the evaporation of the water binder for the sand particles, as well as to any gases produced by a sand core when the latter comes into contact with the liquid metal. These molds have, however, the disadvantage that they are non-permanent, i.e. that they are destroyed in the process of molding or removing the metallic body.

It has been proposed heretofore to provide more or less permanent molds by compounding the molding sand with a binder which is effective for longer periods than the moisture of conventional foundry sands. This binder, which generally was a phenolic resin, vaporized to a limited extent upon contact with the liquid metal so that even these molds had a relatively short life. Other permanent molds were composed of iron (e.g. gray cast iron) and served to produce metal articles from low-melting-point alloys containing zinc, aluminum and brass. These and other metal molds were characterized by a tendency toward chilling the metal introduced so that fine surfaces and high-quality castings could not be reproduced. Additionally, the molds were impervious to the gases generated by the casting process and frequently resulted in the formation of bodies with gas-containing cavities. Another disadvantage of these earlier method molds was that they readily adhered to the cast article, an occurrence which was only partly obviated by the coating of the mold cavities with a ceramic material.

It is an object of the present invention to provide a mold for the casting of liquid material which is both pervious and free from a tendency to adhere to the metal articles while being capable of reproducing articles with high definition.

It is a further object of this invention to provide a casting mold having a high degree of gas permeability and yet of rigid construction with a relatively long life.

Still another object of the invention is to provide a method of making such a mold.

- We have discovered that a mold of exceptionally long life can be produced by sintering together a mixture of metallic particles and refractory particles at a temperature suificient to cause fusion of the particles without, however, closing interstitial passages which communicate with the mold cavity and permit the diffusion of gases away from this cavity. The sintered mixture is disposed at least in the region of the latter although it preferably occupies the entire mold bodies. The resulting metalloceramic body has excellent rigidity and can be formed by compounding the metallic particles with a refractory material in the presence of a binder adapted to impart stiffness to the mixture sufiicient to enable it to be molded and subsequently sintered. The metallic particles may be composed of iron or an iron alloy, with the refractory particles composed of a ceramic or graphite. The ceramic is preferably a high-melting-point metal oxide such as an alumina-containing substance, finely ground quartzite, zirconium oxide or chromium oxide and is present in an amount up to, say, 10% by weight of the mixture, the total amount of the refractory material ranging between substantially 5 and 25% by weight.

It is an essential feature of the present invention that the mold be passive with respect to the cast-ing metal so that a fusion is not possible. This passivity can be insured if the refractory fraction contains from 5 to 15% graphite. The binder may, advantageously, be a phenolic resin which constitutes between 0.2 and 5% by weight of the mixture prior to sintering, although other organic binders decomposable to carbon or even elemental carbon (in the form of charcoal) may be used. The binding effect can be prolonged after the sintering step if the binder is such that it only partly vaporizes during sintering with most of the remainder being released upon contact of the liquid metal with the mold material.

According to still another feature of this invention a compacting of the mixture takes place at an elevated pressure between, say, 500 and 3000 kg./cm. This pressure can be applied prior to, with, or even possibly immediately after sintering mechanically or with the aid of an elevated ambient gas pressure. The pressure can be applied together with the molding step or thereafter without damage to the mold cavity. The mixture may include relatively low-boiling point components, including the binder, which are vaporized to leave voids constituting interstitial passages for the escape of gases. In addition, exothermically active materials can be included so that the heat generated by their reaction supplements that provided for sintering, it is thus possible to carry out the fusion step at a temperature much lower than would otherwise be possible. Suitable exothermic materials include metallic polysulphides and metal oxides capable of reacting with these sulphides to form volatile sulphur oxides. Thus molybdenum disulphide in intimate mixture with metal oxides such as iron and manganese oxides is suitable. The volatile constituent should, however, be present in a particle size less than about microns while the sintered components have a particle size on the order of several hundred microns or greater. The refractory material chosen should be determined by the maximum temperature at which these materials retain their refractory proper-ties. Thus quartzite is capable of withstanding temperatures up to 1700 C., while alumina-containing substances are refractory up to 1800 or 2000 C., while graphite is unaffected by heat up to substantially 3600 C. The mold body thus has a resistance to heat far greater than the temperature of liquid gray cast iron and malleable cast iron.

It should be noted that the present invention also provides for the disposition of ceramic materials in the moldcavity regions of a sintered-metal porous mold body. A mold produced in accordance with the method described above is pervious to the gases normally developed during the casting process as well as those derived from sand cores and is capable of reproducing articles having a high definition. Moreover, the conventional manipulations of foundry sand are entirely avoided so that the casting process can be carried out with recycling of the molds.

The above and other object features and advantages of the present invention will become more readily apparent from the following description, reference being made to the appended drawing in which:

FIG. 1 is a cross-sectional view showing a separable mold provided with a sand core in accordance with the invention; and

FIG. 2 is a perspective view of a system for casting metallic articles with the aid of a mold of this type.

In FIG. 1 there is shown a bipartite mold whose cope half 10a and drag half 10b together define a mold cavity 13 in which a conventional sand core 14 is positioned. A pouring spout 11 and vent 12 communicate between the mold cavity 13 and the exterior while pivot lugs 16 and 1'7 are provided on the mold halves for holding them in position. The mold halves may be formed by casting a mixture produced in accordance with either of the following examples and sintering this mixture at the temperature indicated. The lugs 16 and 17 can be embedded in the mold body during casting and are rigid therewith when the body cools. It should be noted that this arrangement dispenses with the use of the conventional mold bodies although such devices can, if desired, be employed to contain the mold halves in the event that they are to be roughly handled. The mold body can also be constructed for the most part of sintered iron particles with the metal-ceramic mixture disposed only in the region of the cavity.

As can be seen from FIG. 2, a plurality of angularly spaced molds 18 can be positioned upon a turntable 19 which is rotatable in the direction of the arrow 20 to carry these molds past the various stations. The mold halves 10a, 10b can be joined by slotted links 21, 22 which are articulated thereto at the lugs 16 and 17. In the open condition of the mold at station A the sand core 14 is positioned therein. Each mold is then closed and conveyed by the intermittently rotated turntable 19 to the pouring position B at which a liquid metal is introduced into the cavity 13 via the spout 11. The molten metal hardens at the cooling stations C and D, at the latter of which the mold is opened for removal of the article. This arrangement permits the permanent mold to be positioned near the casting apparatus and eliminates transportation of the mold boxes with their molten contents over large distances.

Example I A gas-permeable casting mold for chromium-contain ing aluminum alloys and suitable for the production of door fittings is produced from a mixture consisting of 80 to 85% of said iron powder having a particle size on the order of several hundred microns, 10 to 15 weight-percent powdered natural graphite, 1 to 3% phenolic resin, 1 to 2% bauxite and 1 to 2% by weight of an exothermic mixture consisting of molybdenum disulphide and either iron oxide or manganese oxide in the stoichiometric proportions necessary for complete reaction of the sulphur. It should be noted that the mold mixture contains no chromium since liquid aluminum has a tendency to dissolve materials containing this metal.

The mixture of powders is compacted under a pressure of 500 to 1000 kg./cm. during molding and is subsequently sintered to a temperature of 800 to 900 C. This temperature would not normally suflice for a sintering action in the absence of additional heat provided from the exothermic reaction. The porosity of the metallo-ceramic body is between 15 to The wall of the mold cavity is then treated with graphite. Graphite can also be supplied to the wall of this cavity prior to sintering, e.g. by dusting, so that during the sintering operation it becomes bonded to the remainder of the mold body.

Example II A mold for the production of cast-iron parts is produced from a powdered mixture containing 80 to 85% by weight of a silicon-alloy iron powder, 8 to 12 weight-percent graphite, 3 to 5% chromium oxide (Cr O 1 to 3% phenolic resin and 1 to 3% of a polysulphide/ oxide mixture. The composition is shaped and compressed under a pressure of 800 to 1500 kg/cm. and then sintered at a temperature between 800 and 1000 C. The particles are compressed in a hot state and form a mold body having a porosity of 10 to 20%. The mold-cavity surfaces are dusted with chromium oxide and alumina prior to casting and, advantageously, before sintering.

It will be apparent that the invention described above involves many unmentioned modifications included within the spirit and scope of the appended claims.

We claim:

1. A method of making a casting mold for liquid metals, comprising the steps of homogeneously admixing between substantially to by weight of a metallic powder predominantly constituted of iron, between substantially 1 and 3% by weight of a binder for the particles, between 1 and 3% by weight of an exothermically reactive component consisting of at least one metal oxide and at least one reducing agent therefor in stoichiometrically equivalent quantities for reaction to produce a stoichiometric equivalent of gaseous products, and at least one comminuted refractory material selected from the group consisting of silicon oxide, aluminum oxide, zirconium oxide, chromium oxide and graphite in an amount constituting the balance of the mixture; shaping the resulting mixture to form a mold body provided with a casting cavity at an elevated pressure between substantially 500 and 3000 kg./cm. and sintering said body at a temperature between substantially 800 and 1000 C. to react said oxide and said reducing agent and render said body coherent while forming therein interstitial passages for the escape of gases from said cavity by driving off said gaseous products.

2. A method of making a casting mold for liquid metals, comprising the steps of homogeneously admixing between substantially 80 to 85% by weight of a metallic powder predominantly constituted of iron, between substantially 1 and 3% by weight of a phenolic resin binder, between 1 and 3% by weight of an exothermically reactive compoent consisting of at least one metal oxide and at least one metal-polysulphide reducing agent therefor in stoichiometrically equivalent quantities, and at least one comminuted refractory material selected from the group consisting of silicon oxide, aluminum oxide, zirconium oxide, chromium oxide and graphite in an amount constituting the balance of the mixture; shaping the resulting mixture to form a mold body provided with a casting cavity at an elevated pressure between substantially 500 and 3000 kg./ cm?; and sintering said body at a temperature between substantially 800 and 1000 C. to render said body coherent with a porosity between 10 and 30%, and form therein interstitial passages for the escape of gases from said cavity.

3. A method of making a casting mold for liquid metals, comprising the steps of homogeneously admixing between substantially 80 to 85 by weight of a metallic powder predominantly constituted of iron, between substantially 1 and 3% by weight of a phenolic resin binder, between 1 and 3% by weight of a particulate exothermically reactive component consisting of at least one metal oxide and at least one metal-polysulphide reducing agent therefor in stoichiometrically equivalent quantities and a maximum particle size of microns, and at least one comminuted refractory material selected from the group consisting of silicon oxide, aluminum oxide, zirconium oxide, chromium oxide and graphite in an amount constituting the balance of the mixture; shaping the resulting mixture to form a mold body provided with a casting cavity at an elevated pressure between substantially 500 and 3000 kg./cm. sintering said body at a temperature between substantially 800 and 1000 C. to render said body coherent with a porosity between 10 and 30%, and form therein interstitial passages for the escape of gases from said cavity; and coating the wall of said cavity with a layer of a refractory material selected from the group which consists of graphite and high-melting-point metallic oxides.

4. The method as defined in claim 3 wherein said layer is an oxide selected from the group consisting of aluminum oxide and chromiumoxide.

(References on following page) References Cited by the Examiner UNITED STATES PATENTS Mott 75214 Krapf 75206 Brundin 75213 Ehrlich et a1. 106--38.9

Mouroen 75222 Meyer-Hartwig 75206 6 3,158,473 11/1964 Gatti 75206 FOREIGN PATENTS 1,110,368 7/1961 Germany.

696,301 8/1953 Great Britain. 701,541 12/1953 Great Britain. 714,560 9/ 1954 Great Britain.

MARCUS U. LYONS, Primary Examiner. 

1. A METHOD OF MAKING A CASTING MOLD FOR LIQUID METALS, COMPRISING THE STEPS OF HOMOGENEOUSLY ADMIXING BETWEEN SUBSTANTIALLY 80 TO 85% BY WEIGHT OF A METALLIC POWDER PREDOMINANTLY CONSTITUTED OF IRON, BETWEEN SUBSTANTIALLY 1 AND 3% BY WEIGHT OF A BINDER FOR THE PARTICLES, BETWEEN 1 AND 3% BY WEIGHT OF AN EXOTHERMICALLY REACTIVE COMPONENT CONSISTING OF AT LEAST ONE METAL OXIDE AND AT LEAST ONE REDUCING AGENT THEREFOR IN STOICHIOMETICALLY EQUIVALENT QUANTITIES FOR REACTION TO PRODUCE A STOICHIMETRIC EQUIVALENT OF GASEOUS PRODUCTS, AND AT LEAST ONE COMMINUTED REFRACTORY MATERIAL SELECTED FROM THE GROUP CONSISTING OF SILICON OXIDE, ALUMINUM OXIDE, ZIRCONIUM OXIDE, 